Production of alloy-surface castings



' have PM Au 20, 1929.

UNITED STATES ROGER WILLIAMS, 0! WILMINGTON, DELAWARE, ASSIGNOB, BY KESNE ASSIGN- MENTS, TO ELECTRO METALLURGICAL 'GINIA COMPANY, A CORPORATION 01' WEST VIB- PRODUCTION OF ALLOY-SURFACE CASTINGS.

No Drawing. Application filed February This invention relates to metal castings having a body of a base metal, for example iron or steel, and a chromium-alloy surface or overlay upon and forming a unitary part of the body. Such castings while oflow cost compared to a casting composed throughout of chromium alloy, have at their surfaces the properties characteristic of the chromium alloy and therefore, notwithstanding their lower cost, are admirably adapted to serve the purposes of all-alloy castings. Such allalloy castings although referred to for purposes of comparison are, in the case of large castings which may weigh thousands of pounds, commercially impractical, since their cost is absolutely prohibitive. The present castings not only serve in the place of small all-alloy castings but also fill the place entirely unable to be filledby large castings of that type.

In the production of alloy-surface castings the desire, indeed the practical necessity, is to produce a casting with a proper depth of overlay, which overlay cannot easily be chipped or sealed off, nor separated from the body casting upon bending or breaking. It is also important to produce a casting the coating of which, under the conditions to be met with in use, will not diffuse or be absorbed into the metal of the base casting. I devised a method of producing chromium-alloy-surface castings which satisfactorily meet the stated, and other, desirable conditions. It is a major object of the invention to provide such a method, and to this end, and also to improve generally upon methods and castings of the character indicated, the invention consists in the various matters herein described. and claimed. One of the features of particular merit in the accomplishment of these ends is the use during the casting operations of a uniformity-promoting or oxide-dissolving and fusion-pro moting agent which ensures the production of a completely fused and homogeneous coating, as willbe discussed more fully in the course of the detailed description of the invention now to follow.

The present general procedure comprises coating the surfaces of the mold, corresponding to those surfaces of the casting on which the alloy coating is desired, with a layer of coating material containing chromium with or without the admixture of other metals, or of suitable particular composition to produce 13, 1928. Serial I'm 88,189.

the particular type of casting desired, and pouring the base metal in the coated mold. The coating may be varied from inch or less to inch or more in thickness. The base casting metal, on flowing intothe mold, flows over and melts or fuses the coating material in place, the heat of the base metal causing the chromium alloy to fuse or run together, with more or less considerable admixture of base casting metal, forming a plastic or semi-fluid mass which is held in place by the pressure of the molten metal in the mold, and which sets or solidifies with the base casting itself, the two forming a definite solid body. In this procedure, there is preferably used, to

hold the granular or coarsely pulverous chromium in position on the mold, causing it to adhere thereto, a suitable binding agent, with which the chromium is desirably mixed; and various materials exemplified by silicate of soda have heretofore been suggested for use asbinding agents in procedures involving the general feature of coating the mold.

It has been found in the practice of the general method indicated that it is frequently not fully satisfactory for the production of high grade coatings and castings, since castings with their coatings not properly uniform are often produced. A probable reason for this is, I believe, that the hot casting metal when it enters the mold has a chemical action upon the particles of crushed coating material, by which a film of oxide is formed on these particles; and this oxide tends to prevent the rapid and complete fusion necessary for the formation of a uniform coating.

Now I have found that the use of certain substances in connection with the coating material results in the production of satisfactory coatings. These substances are metal fluorides, e. g. sodium fluoride and calcium fluoride, also borates or the like as borax or boric acid, or compounds or mixtures containing these substances, also cryolite. Such materials apparently dissolve oil", or prevent the formation of, the mentioned film of oxide. Whatever the explanation, they result in a satisfactory coating. In general, then, I term these substances uniformity promoters, oxide preventers and oxide dissolvers. They are desirably used to the extent of .5 to 5.0 per cent by weight of the coating material: that is to say to a given weight of coating material there is added a weight of agent equal to .5 to 5.0 per cent of the said given weight. Preferably the agent is mixed directl with the-coating material in a finely divi ed state. I have found by experiment as best, and prefer of these materials, sodium fluoride. This is referably used in powdered condition and 111 the proportion of 1 to 2.5 per cent by weight. It may be added to the crushed coatin material previous to the introduction of the inder, e. g., previous to the mixmg of the binder with the coating material. While inorganic binders, efg. sodium s1l1cate, may be used, I much prefer an organic binder such. as molasses, glucose, linseed o1l containing 10 to 20 per cent of dissolved rosin, etc., and have found that, enerall speaking, for a chromium alloy sur ace of t e type desired, the coating material should be crushed to ground to a size such that all the particles will pass through a mesh screen and substantially all be retained on a 50 mesh screen. For eral purposes I prefer to use material size to to mesh. For special purposes material even as fine as 100 mesh maybe used; as for example the purpose indicated in U. S. Patent No. 1,545,438, Mitchell, July 7, 1925, wherein material of this fineness is used for coating over a layer of coarse material in order to ensure that during the pouring the. coating shall not be washed away.

As a specific example of procedure in accordance with the invention the following will serve:

A mold fora steel or iron casting is repared in the customary way and the sur ace of this mold, while in a green state (before baking or drying) is coated to the proper depth, say 1/8", with granular chromium coating material containing 2.0% of sodium fluoride and sufficient binding material, as linseed oil, molasses, etc. of the kind specified, to form a plastic mass or mortar. This is applied with the molders trowel evenly and in a layer of uniform thickness all over the mold surface that it is desired to coat. With a mixture of proper consistency each square foot of coating 1/8" thick will contain from 1 to 1 lbs. of coating material.

The binder causes the coating to adhere to the sand mold surface; and after drying according to usual foundry practice, and baking to a temperature of 600 to 700 F. (the usual temperature for baking molds for iron or steel), the crushed coating material mass is so hardened and adherent to the surface of the mold that it may be brushed over by the hand without displacing or dislodging it.

p The mold having been thus prepared, the base metal is poured into the mold and the casting made. Desirably, though not necessarily, the casting may be done in such manner as togive a high ferro-static pressure in the mold, as by use of a high pouring ate; e. g. a gate extending to 20 to 30 inches a ove the top of the mold cavity, and pouring rapidly, e. using with a gate 2 to 3i11ches in diameter a pouring speed ofIOOFp nnds or more per 5 seconds, indicating approximately 4 pounds per second per square inch' of area. The pouring temperature of the.

for the coating material, rather fthan, say, a

ferro-chromium alloy, temperatures .above 3000 F. are advisable.

While, as indicated above, the coating ma terial (together with sufficient binder to form a soft paste of the consistency of ordinary mortar, and oxide-dissolver) may be applied directly to the surface of the mold with 1 an ordinary or slicked of fabric, such as wire gauze or textile material, of fineness and closeness of mesh sufficient to hold the crushed chating material in place may be coated with a mixture of crushed chromium alloy, with or without other metals and oxide dissolving material, and binder to the roper thickness, the coated sheet dried and tted, and fastened to the molders trowel and smoothed into place, the coating may be .1 "applied in other ways. For instance, a sheet mold or core surface as by chaplet nails, wire staples, or similar devices which will insure the coated sheet remaining in proper position and undisplaced by the molten metal while the mold is beingpoured. If wire gauze is used as a supportin material for the crushed coating material, t is should be made of a metal, say iron, which will be fused by the molten casting metal and become incorporated with the casting or coating when the mold is being poured. In case a textile material is used, it may be ordinary cheese cloth or a similar light weight material.

Or, as another method, the coating may be sprayed on the surface of the mold or core,

which may be accomplished by mixing the finely divided coating material and oxide dis.- solvmg material and forcing the mixture from a suitable nozzle by compressed air, the

binder separately introduced, being mixed with it e said mixture at the moment of themixtures ejection from the nozzle. As

another method, the dry coating material oxide dissolvin materlal ma be spread over the surface of t e mold and eld in place by spraying with the binding material.

either case the whole is subsequently dried before the molten casting metal is introduced into the mold. The junctions of such sheets or any curved surfaces are coated with the trowel by hand so as to make the whole mold coating continuous and uniform in thickness. Either the spraying method or the coatedsheet method are particularly suitable as a means of saving time for coating large plane surfaces, while for more intricate molds or cores, hand coating with trowel must necessaril be used.

I have found by experiment that chromium metal in various forms and combinations may be used as a coating material:

either commercially pure metallic chromium containing only small proportions of iron, carbon, or other impurities, or alloys varying through the series of ferro-chromium alloys to that known as maximum 8% carbon ferro-chromium which contains ap proximately 65% to 70% chromium and 6% to 8% carbon, the balance iron and impuri ties. From a practical standpoint, however, as regards cost and quality of coating produced, I prefer to use the ordinary commercial ferro-chromium known as maximum 6% carbon which contains approximately 65% to 70% chromium, 5% to 6% carbon, the balance iron and impurities. I find that coatings made with such material according to my process show practically complete oxidation resistance at temperatures up to 1800 to 1900 F.

Not only is the invention applicable to base castings of steel, and of steel alloys as above indicated, but also is applicable to base castings of cast iron. As a rule, with cast iron as the base metal the coating will be only partially fused and the particles imbedded in a complex matrix of iron and steel. Such coatings, in view of the very great hardness of the ferro-chromium, will have marked abrasion resistance, both for not and cold work and when used as a coating for the inside of ingot molds for steel will prevent to a considerable extent, the cutting action of the hot metal during pouring.

As of interest I may state that metallographic examination of the coatings produced on steel castings by my process show that the coating or overlay is a separate and distinct layer or veneer which is firmly adherent to the surface of the casting. With 'the use of low carbon steel as the base metal the coating is generally of a hypo-eutectoid character in that there is a predominance of iron-chormium solid solution with a minimum of iron-chromium carbide present. With the use of high carbon steel the character of. the coating will be altered, and owing to more or less absorption of carbon from the steel the coating will have a hyper-eutectoid character in which there is an excess of iron-chromium carbide. As a general rule the hypo-eutectoid coatings will be found to have greater oxidation and corrosion resistance than the h per-eutectoid, while the latter owing to t e greater predominance of iron-chromium carbide will have greater abrasion and wear resisting qualities. Chemical analysis of coatings made from 6% maximum carbon ferro-chromium will show an average chromium content of over 35%.

I Photomicrographs of sections of steel castings which have had upwards of 2000 hours service at a high temperature show the coating still a distinct and separate layer adherent to the base metal with no appearance that would indicate diffusion to have taken place. I

I claim:

1. A coating for molds .adapted to form a homogeneous surface material alloyed with a base-metal cast in the mold which comprises metal particles containing chromium,

, a woven organic material, and a material capable of inhibiting the formation of oxides and of promoting the uniformity of the coating.

2. homogeneous surface material alloyed with a base metal cast in the mold which comprises particles of ferro-chromium and a material capable of inhibiting the formation of oxides and of promoting the uniformity of the coating.

3. A coating for molds adapted to form a homogeneous surface material alloyed with a base-metal cast in the mold which comprises particles of ferro-chromium and a material comprising a chromium-oxide-dissolving fluoride.

4. A coating for molds adapted to form a homogeneous surface material alloyed with a base-metal cast in the mold which comprises particles of ferro-chromium and a material comprising sodium fluoride' In testimony whereof I afiix my signature.

ROGER WILLIAMS.

A coating for molds adapted to form a I 

